Electrical distribution networks are critical for the delivery of electricity to consumers and businesses from the transmission system. Such a network can include power lines, substations, transformers, and meters that are interconnected by thousands of miles of cables. In densely populated urban areas, electricity is transmitted to consumers via secondary low-voltage AC network systems which are formed by feeding several transformers into a common bus. A secondary low-voltage AC network system is generally characterized as a system wherein customers are served from three-phase, four-wire low-voltage circuits supplied by two or more network transformers whose low-voltage terminals are connected to the low-voltage circuits through network protectors. The secondary network system has two or more high-voltage primary feeders, with each primary feeder typically supplying 1-30 network transformers, depending on network size and design. Such systems include automatic protective devices intended to isolate faulted primary feeders, network transformers, or low-voltage cable sections while maintaining service to the customers served from the low-voltage circuits. While secondary low-voltage AC network distribution connections as discussed herein are typically housed in an above-ground cabinet or a below-ground box, it is generally preferred that these secondary networks are located underground, due to the impracticability of using overhead wires in densely populated urban areas. As a result, the connections may be subjected to moisture and may even become submerged in water. If the cable conductors or conductor members of the bus bars are left exposed, water and environmental contaminants may cause short circuit failure and/or corrosion thereon. In addition, as an urban area grows, the process of connecting additional customers to the secondary network involves the costly procedure of excavating and splicing connections to the common bus. Such problems of expansion have not been entirely ignored by the industry.
It is believed that since about 1935, crab joints have been used for reducing the time associated with interconnecting mains cables used in underground secondary low-voltage AC network systems. A crab joint basically includes a central hub (often referred to as a “busbar”) with multiple fusible connections (referred to as “limiters”) to a number of cables constituting part of a network. The limiters act to protect the cables connected to the crab joint in case of a failure of any of the cables in the network. A typical crab joint consists of a plurality of cable connectors, also known as legs, electrically connected to a common junction plate. The connectors are arranged such that a plurality of legs surround a center leg as shown in FIG. 1A (PRIOR ART). The center member is connected to other electrical components, such as another crab joint, while the surrounding legs are connected to a continuation of the cables for “network mains” or feeding customers. More specifically, FIG. 1A depicts the typical arrangement wherein a center connector 100 is surrounded by a plurality of connectors 102. Connector 102 is typically a fusible connector, while connector 100 is non-fusible and connected to other accessories, such as another crab joint. The crab joint greatly reduces the labor associated with splicing cables and improves reliability because the linesperson simply connects a prepared cable to an available connector, instead of preparing multiple cables for splicing. This achieves the requirement to connect a number of cables to one geometric point (in electrical terms).
The conventional crab joint generally used in secondary low-voltage AC network systems comprises compression connectors with EPDM rubber seals to connect network cables to the busbar. In older prior art joints, the limiter elements could not be individually replaced and in early conventional crab joint design, a failed or blown limiter was not readily discernible from the exterior of the crab joint. Of course, such construction made it very difficult to visually detect an opened limiter in a crab joint. As a result, opened limiters often existed undetected for long durations resulting in low voltage in the serviced area or overloading of a network transformer. Repairs of such problems required excessive work by “troubleshooting crews” tasked with testing and inspecting substantial lengths of cable runs and related components without an easy solution for troubleshooting failure.
While there have been several improvements to the crab joint, such as material improvements, fault identifiers, and the like, the arrangement of the connectors has remained largely unchanged since the original development of crab joints over seventy years ago. One major development in the field of secondary voltage systems was the introduction of fusible connectors; however, crab joint configuration design still generally remained the same. Fusible connectors were designed in response to one or more mains in a crab joint experiencing excessive currents due to a short circuit. The short circuit fault caused the connector of a crab joint to overheat and eventually destroy itself. This destruction could subsequently damage the surrounding connectors and mains cables that may not have been experiencing the fault current. As a result, a short circuit fault could bring down power to all customers connected to the crab joint. A limiter, such as a fuse, is typically located between the mains connection and junction plate—where all the mains connectors are electrically connected together. The limiter is designed to disconnect the main experiencing the fault condition, just before the heat from the short circuit current damages the other mains connected to the crab joint.
One primary reason that crab joint design has not departed from the original design conceived over seventy years ago is that current crab joint designs simply work for the intended purpose. As referenced, even with the addition of the fusible connectors, crab joints have been modestly modified and maintained the same arrangement for the connectors. In fact, the addition of these limiters has reinforced the existence of the current design, as the minimum distance between connectors must be such that it impedes a blown limiter from damaging other connectors. Thus, rather than minimizing the distance between crab joint connectors, a current trend of crab joint designs focuses on improving fusible connectors to exist in the environment of existing crab joints. For example, Mofatt U.S. Pat. No. 7,358,845 entitled “Cable Limiter and Crab Limiter Employing Replaceable Fusible Element” is directed to the improved fusible connectors and references various other inventions relating to improving fuses which are designed to be compatible with crab joint systems. FIG. 1B depicts a crab joint known in the art which is integral to the operation of the improved design of fusible connectors of Mofatt. As shown, the connectors are arranged similar such that a center connector 104 is surrounded between a plurality of connectors 106. It is readily apparent in the Moffat disclosure that the crab joints known in the art are of the typical design disclosed in FIG. 1A wherein the ring bus leg is disposed in the center of the arrangement. In the Mofatt design, when a fusible connector fails, only the failed fuse needs to be replaced instead of the whole crab joint. While Mofatt teaches the inclusion of an annunciator on the fusible connector to indicate the status of the fusible element (wherein the annunciator can either be an auditory signal or wireless communication in order to facilitate personnel in finding and replacing blown fuses), the invention of Mofatt is designed to be compatible with the connectors in the traditional crab joint formation currently known in the art.
Even when inventions in the field of crab joints result in the adjustment of the configuration of crab joints, the change is minimal and is generally for purposes other than compactness. In one example of modified crab joint design (U.S. Pat. No. 8,129,618 entitled “Cable Joint,” issued to Bier), the location of the center connector is offset. According to the patent, this modification was primarily to facilitate a U-shape which was designed to allow personnel to easily view and visually identify a blown fuse without having to move the location of connected components. This modified design is shown in FIG. 1C (PRIOR ART). Specifically FIG. 1C depicts a preferred embodiment of Bier, which teaches a seven way crab joint with an improved design including a visual indicator for determining whether or not a corresponding limiter is blown. Center connector 108 is positioned between a plurality of connectors 110, but is offset from a central position of the crab joint. As mentioned, the purpose of the change of the location of the center connector was to facilitate personnel easily looking down and identifying a blown fuse, rather than for the purpose of providing a compact design which eliminates material and space requirements when in use.
While FIG. 1C discloses the improved seven-way crab joint of Bier U.S. Pat. No. 8,129,618 entitled “Cable Joint,” a simpler seven-way design is presented in FIG. 1D. Specifically, FIG. 1D illustrates an arrangement that adapts the arrangement in FIG. 1A, but for a seven way crab joint. Similar to the arrangement in FIG. 1A, a center connector 112 is surrounded by connectors 114.
It is well known in the art that space allocation in electrical component cabinets and underground systems is a primary concern, especially as more power is necessary to support residential and commercial areas in growing metropolitan cities. The facilities for the components of secondary low-voltage AC network systems is generally not expanding in proportion to the demand, and as a result, the components themselves need to change. Thus, there is an apparent need in the art for components of reduced size. However, merely altering the size of the components is not always an easy solution. In particular, crab joints must be designed of sufficient size, shape, and material so that the crab joint can handle the substantial voltage without significantly impeding current flow, can be easily repaired, can be compatible with existing systems, and most importantly can handle surges and potential damage from blown fuses. So while there exists an apparent need for a compact crab joint design, no known developments have resulted in an improved design that meets the objectives required to operate in the complex environment of secondary low-voltage AC network systems.